To make clinical assessment regarding the role of Helicobacter pylori (H. pylori) in the occurrence of gastric cancer, we examined serological and microbiological positive rate of H. pylori and histological and endoscopic findings of atrophic gastritis of background mucosa in 149 gastric cancer patients, 136 gastric ulcer patients, 82 chronic gastritis cases and 46 normal control. Serological H. pylori positive rate examined using GAP-IgG in 43 gastric cancer patients was 95%, which was significantly higher than normal control (4%) but the same level as in chronic gastritis cases (87%) and gastric ulcer patients (93%). While, Microbiological H. pylori positive rate by culture was 62% in 149 gastric cancer, 7% in normal control, 74% in chronic gastritis and 80% in gastric ulcer. H. pylori infection and the degree of lymphocyte infiltration were significantly correlated and H. pylori was frequently detected in the cases with mild and moderate atrophic gastritis in all groups. No difference was observed in H. pylori positive rate between the well and poorly differentiated cancer cases; 96% and 94% by GAP-IgG, 57% and 66% by culture, respectively. As a conclusion, it was suspected that the relation between H. pylori infection and gastric cancer was indirect and associated with atrophic process of background mucosa.